(a) Field of the Invention
The present invention relates to an embroidering sewing machine and, more particularly, to an embroidering machine for altering an embroidering stitch type in dependence on a size of a pattern to be formed.
(b) Description of the Prior Art
An embroidering machine for forming a pattern by arbitrarily altering the size of the pattern selected from a plurality of different patterns stored in a memory in a form of pattern information, is generally known.
In an embroidering machine which can alter the size of a selected pattern stored in advance, when the size of the pattern is altered or adjusted beyond a critical size, it may happen that an embroidering frame carrying a fabric to be sewed will not move in synchronism with a vertically reciprocating needle, especially when the pattern is to be formed with a so-called type of satin stitches shown in FIG. 7(a), that is, zigzag stitches with a close feed pitch.
In this case, a special mechanism for disconnecting the needle bar from the drive shaft of the sewing machine is used.
The needle bar remains disconnected until the fabric holding frame is correctly positioned under the needle. However, since the fabric holding frame is required to move a distance with respect to the needle stopped in a position above the fabric, the thread may get twisted, loosen, or may get caught by the needle point and damaged or cut off. In any event, it is difficult to obtain a desired seam; in particular, the satin stitches shown in FIG. 7(a) when the adjusted size of the selected pattern is beyond a predetermined critical size.
In a conventional embroidering machine that does not have a mechanism for disconnecting the needle bar from the drive shaft, the fabric-holding frame is displaced in a plurality of steps with each step synchronized with the vertical reciprocation of the needle of a set amplitude when the set amplitude is beyond a critical amplitude, so that mat-type stitches, shown in FIG. 7(b), may be formed.
However, if a section of satin stitches and a section of mat-type stitches are mixed in one pattern, the appearance of a sewn pattern is adversely affected.
On the other hand, when the size of the pattern is altered to a degree which is smaller than the critical size of the pattern the sewing width of the pattern is continuously narrowed until the stitches become substantially straight stitches dependent on how the adjustment was effected.
In this case, the needle is liable to repeatedly drop relative to the same position of the fabric. As a result, the needle will damage the seams and often will cut the seams. Japanese Patent Application 61-33788 of the applicants herein discloses a method for automatically enlarging a stitch width in such a case. However, this is different from the invention of the subject application which relates to forming large and small size patterns.